Patent GB-1046024-A (GB)
Title: Improvements relating to guidance apparatus
Inventor: JONES SIDNEY
IPC: E02F5/14
Filed: 1962-10-09
Abstract: 1,046,024. Optical guidance apparatus. R. B. PULLIN & CO. Ltd. July 25, 1963 [Oct. 9, 1962], No. 13594/62. Heading H4D. Optical guidance apparatus comprises means at the object or fixed position, producing a cyclically pulsated beam of light such that light detectors at the fixed position or object can determine their position from the modulation of said beam. Fig. 1 (not shown) shows apparatus, whereby a movable object 3 can be guided along a straight line towards a fixed position, comprising a transmitter 1, which may be mounted as shown at the fixed position, or alternatively on the moving object, and a receiver 2, correspondingly mounted on the moving object or at the fixed position. The transmitter, Fig. 2 (not shown), may comprise a light source 9, producing light which is directed by a condenser 10 on to and through a reticle 11, Fig. 3 (not shown), comprising alternate transparent and opaque sectors. A lens 12 is mounted for rotation by a motor 19 about an axis 13 collinear with the axis of the reticle but offset from the optical axis of the lens. The optical axis of the lens therefore describes a circle over the reticle. A third lens 30 collects the light from lens 12 and directs it as a slightly diverging beam towards the receiver. At any point within the cross-section of the beam a series of light pulses will be received having a pulse repetition rate varying in dependence on the position of the point within the cross-section of the beam. The light variation can be imagined as being produced by scanning the illuminated reticle in a circular path having a centre depending on the said position of the point. The receiver, Fig. 4, comprises two photo-cells 31 and 32 mounted one above the other, having outputs with instantaneous frequencies given by:- where f m = mean chopping frequency, r, a = polar co-ordinates of photocell position, with respect to the centre of the reticle. R = radius of circle which the photo-cell can be considered as tracing out over the reticle surface. The outputs of the photo-cells are passed to discriminators 33 and 34 which give outputs in the form:- which, in complex vector form, with the x axis taken as a line parallel to that joining the two photo-cells and passing through the projection of the optical axis of the lens 10 and reticle 11, becomes Thus if x 1 , y 1  are the co-ordinates of photo-cell 31, x 2 , y 1  the co-ordinates of photo-cell 32, and x, y, the co-ordinates of the mid-point 46, then the outputs of discriminators 33 and 34 become respectively:- These ouputs are added and subtracted in means 35 and 36, respectively, to give respective outputs of:- where D is the distance between the photo-cells. =X 2 -X 1 . Equation (6) represents a signal of the form 2V -.r.cos(α - wt), and equation (7) represents R VD a signal of the form - cos wt, which becomes R VD - sin wt after a 90 degrees place shift at 43. R The signals represented by equations (6) and (7) are passed via variable gain amplifiers 37 and 38, to be explained later to X and Y coordinate detectors 41 and 42 respectively. The two signals are multiplied directly in X co-ordinate detector 41 to give an output representing X, whilst the signal of equation (6) is multiplied in Y co-ordinate detector 42 by the signal - VD/R sin wt, from 90 degrees phase shift 43 to give an output representing Y. The two output signals therefore represent the position with respect to the axis of the reticle of the midpoint 46 of the two photo-cells, and can be fed to indicators or to servo drives to cause the movable object to travel on a straight line towards the transmitter. Due to the presence in the equations (6) and (7) of the amplitude term V, the gain of the amplifiers 37 and 38 must be varied to keep this term constant irrespective of its variation at the inputs to the amplifiers due to the varying distance between the transmitter and receiver. Such control is produced by means of a feedback gain control signal, fed from the output of amplifier 38, to the gain controls of the amplifiers, via an amplitude detector and low pass filter. In an alternative embodiment, Fig. 6 (not shown), the reticle may be as shown in Fig. 5 (not shown) with the transparent sectors 50 in one half of the reticle more transparent than those 51 in the opposite half. The light now received by a photo-cell at the receiver is thus in a series of pulses, with the pulses having a varying amplitude as the two halves of the reticle are scanned as well as the previously explained varying repetition rate. The receiver now comprises a single photo-cell feeding its output to an amplitude demodulator 52 and a frequency demodulator 53 in parallel. The output of the frequency demodulator is fed to phase sensitive rectifiers 55 and 58, which are respectively controlled by the output of the amplitude demodulator and by the said output phase shifted by 90 degrees in a quadrature generator 57. The output of phase-sensitive rectifier 55 is then indicative of the vertical position of the photo-cell, whilst the output of the phasesensitive rectifier 58 is indicative of the horizontal position of the photo-cell. An alternative method of producing an effective rotation of the optical axis of the lens 12 over the reticle is shown in Fig. 7 (not shown), wherein two prisms 65 and 66 are mounted with the reflecting surface of prism 66 facing the outer region of the reticle, and the reflecting surface of prism 65 facing the projecting lens 63. The prisms are mounted for rotation together about the optical axis of the lens 63, such that the said optical axis scans a circular path over the outer region of the reticle. In further alternatives the effective rotation may be produced by an actual nutation of the reticle, or a negative lens may be eccentrically mounted for rotation about the optical axis of the reticle and projecting lens, between the said reticle and projecting lens. To increase the range of the apparatus, infra red radiation may be used. The apparatus may be applied to guiding a beam riding missile. The reticle may be photo-etched from a stainless steel plate and may be in the form of an Archimedian spiral.